One pot, steeper lowpass filter.

[mills]

So, I've got a build or two that I'd like to include a lowpass filter on.

I want steeper than the usual cap to ground (6dB per octave?), but I'm not necessarily sure exactly how much.  I'd like to be able to vary the cutoff frequency from high enough to not be too apparant, to pretty muffled, and I'd like to be able to do it with one pot.  Preferably a single pot, but maybe a double if there's no other way.

I don't mind reading/searching, but I know there's a bunch of different active filter topologies and I don't know the names.  Is there a setup or two that could do what I want, or am I asking the impossible?  Even a suggestion of something sort of in the ballpark would be appreciated so I can look into it.  I'm hoping for 1/2 to a whole dual op amp, and will be using it in an effect or two thats run at +/- 12 or 15V if that helps at all.

Thanks!

[Jarno]

In principle, it's a resistor and then a cap to ground, and you could add another stage. But it's better to use a second order sallen key, that's 12dB/ octave. You will need a stereopot though. But if you make the cap to ground switchable you can influence the Q factor, which adds "character" (at higher Q's).

[mills]

Thanks! That's pretty simple, and probably worth using the stereo pot.  Lead to some decent explanations of other filter types as well.

[R.G.]

Active Filter Cookbook, Don Lancaster

[mills]

Again, thanks!  I was secretly hoping for a pdf when I googled it, but its probably worth picking up a copy.

[Jarno]

Absolutely! I got one secondhand for very little cash a while ago and everything you need to know (and then some) is in there. Not the easiest of books though, not a lot of complete practical schematics.
This is a nice online calculator

[mills]

Yeah, I think that the calculator will help get me rolling with an effect in the short term, which is good.  Getting through some of the theory and doing a bit of math at some point should be good for me.

[isildur100]

This calculator is also very handy:

http://astro.u-strasbg.fr/~koppen/radio/Activee.html